Torque transmitting mechanisms such as clutches and brakes employ a hydraulically operated piston to enforce frictional engagement between a plurality of friction plates that are alternately splined to a housing containing the piston and a hub member that is generally connected with a gear member. When the torque transmitting mechanisms are inactive during vehicle operation, a cavity or chamber formed between the piston and the housing remains filled with very low pressure oil. However, when the vehicle is stopped and the engine is not operated for a period of time, such as overnight, it is possible for the hydraulic fluid in the cavity to drain to the sump and the cavity becomes filled with air.
On a subsequent start-up of the vehicle, the air must be expelled from the cavity before consistent shift quality is attained. This may require five or more shift cycles which are objectionable. With today's electro-hydraulic controls, consistent hydraulic fluid fill times and volumes are necessary for shift quality consistency. The entrapped air in the cavity prevents the required consistency. The electronic controller of the electro-hydraulic control uses the data from the previous shift to calculate the optimum flow and pressurization rates to be employed during the current shift event. Since air is a compressible medium and hydraulic fluid is a relatively incompressible medium, if air is present in the cavity, it is difficult for the controller to determine the optimum flow and pressurization rates. During the first five or more shift events, the air is slowly bled from the cavity under the piston seals until the cavity is filled with hydraulic fluid only.
Some currently available power transmissions incorporating hydraulically operated torque transmitters utilize ball bleed valves and other such devices to provide a controlled passage through which the entrapped air can be evacuated. These devices often allow a significant amount of hydraulic fluid to leak from the cavity after the air bleed function is complete. The repeatability of these devices is inconsistent due to the variation in oil viscosity due to the operating temperature changes in the transmission.